In the oil and gas industry, oil producers access sub-surface hydrocarbon-bearing formations by drilling long bore holes into the earth from the surface. Conventional drilling comprises advancing a rotating drill bit through the hole, the bit being mounted on a bottom hole assembly at the distal end of a drill string. During drilling, friction between the downhole assembly and the earth can impair the rate of penetration in the hole. In particular, where highly deviated holes or horizontal holes are being drilled, the weight of the drill pipe alone cannot be relied upon to overcome friction from the string resting against the wall of the hole.
One means for overcoming downhole friction is to impart a vibration or movement to the drill string. For example, the “AG-itator” tool disclosed in U.S. Pat. No. 8,167,051 comprises the use of a 1:2 lobe Moineau principle positive displacement motor (PDM) to control a valve arrangement that oscillates in and out of alignment as the pump snakes back and forth. Oscillation of the valve arrangement causes an increase in fluid pressure (as the valve closes) and corresponding release of pressure (as the valve opens), creating a pressure pulse capable of vibrating the string. The pressure pulse magnitude and frequency of such tools, however, are limited by the tool design. Other conventional tools operate by creating backpressure in the fluid supply. These tools require supply pumps of greater capacity and also reduce the supply pressure to the drilling bit.
U.S. patent application Ser. No. 13/381,297 teaches a “Rattler” vibration tool that induces movement of the string by reducing the overall fluid pressure within the drill string, creating a negative pressure pulse. In the Rattler tool, drilling fluid is pumped down the drill string and then cyclically vented from the tool to the annulus through a fluid port disposed in the side wall of the tool. This tool, however, teaches the use of a turbine-type rotor in the tool body, resulting in a limited size and frequency pressure pulse that can be achieved (that is—venting of fluid from the tool is limited to the available fluid pressure that can be vented, and the corresponding pressure drop directly correlates to the uncontrolled speed of the “spinning” turbine-type rotor).
Known vibration tools are not, capable of providing controlled, tunable pressure fluctuations, that is—the magnitude and frequency of pulses created by known tools is fixed according to the size and capacity of the tool. Other known tools are also often reliant upon downstream pressure losses and are unable to create a sufficient vibration where downstream pressure is low.
There is a need for a downhole vibration tool that is capable of providing a higher magnitude controlled pressure pulse, enabling operators to dictate the intensity and frequency of the vibration, without the need to modify the fluid flow rate through the tool and without any reliance on downstream fluid pressures.